1. Field of the Invention
The present application relates to a compressed natural gas injector which incorporates an improved low restriction valve needle seat to control the fuel flow in the needle valve seat area.
2. Description of the Related Art
Compressed natural gas (hereinafter sometimes referred to as xe2x80x9cCNGxe2x80x9d) is becoming a common automotive fuel for commercial fleet vehicles and residential customers. In vehicles, the CNG is delivered to the engine in precise amounts through gas injectors, hereinafter referred to as xe2x80x9cCNG injectorsxe2x80x9d. The CNG injector is required to deliver a precise amount of fuel per injection pulse and maintain this accuracy over the life of the injector. In order to maintain this level of performance for a CNG injector, certain strategies are required to help reduce the effects of contaminants in the fuel and to control the flow of fuel through the injector.
Compressed natural gas is delivered throughout the country in a pipeline system and is mainly used for commercial and residential heating. While the heating systems can tolerate varying levels of quality and contaminants in the CNG, the tolerance levels in automotive gas injectors is significantly lower. Accordingly, utilizing CNG in engines presents problems unique to CNG as well as to the contaminant levels.
These contaminants, which have been acceptable for many years in CNG used for heating affect the performance of the injectors to varying levels and will need to be considered in future CNG injector designs. Some of the contaminants found in CNG are small solid particles, water, and compressor oil. Each of these contaminants needs to be addressed in the injector design for the performance to be maintained over the life of the injector.
The contaminants can enter the pipeline from several sources. Repair, maintenance and new construction to the pipeline system can introduce many foreign particles into the fuel. Water, dust, humidity and dirt can be introduced in small quantities with ease during any of these operations. Oxides of many of the metal types found in the pipeline can also be introduced into the system. In addition, faulty compressors can introduce vaporized compressor oils which blow by the seals of the compressor and enter into the gas. Even refueling can force contaminants on either of the refueling fittings into the storage cylinder. Many of these contaminants are likely to reach vital fuel system components and alter the performance characteristics over the life of the vehicle.
In general, fuel injectors require extremely tight tolerances on many of the internal components to accurately meter the fuel. For CNG injectors to operate on CNG while remaining contaminant tolerant, the guide and impact surfaces for the armature needle assembly require certain specifically unique characteristics.
The CNG injector is required to accurately inject metered pulses of fuel over the life of the injector. It is also necessary to be able to calibrate the injector to a specific calibration. Before it is possible to calibrate a CNG injector, the design must have solved many of the specific problems inherent in using CNG, including higher fuel pressures and needle lift when compared to a standard gasoline injector, choked sonic flow, and pressure losses through the injector. For proper calibration of the injector, the two most important parameters which require control are pressure upstream of the choked flow, and orifice size.
In addition, to problems of contaminants in gaseous fuels, other problems relating to flow conditions and pressure losses must also be addressed. For example. whereas in a standard gasoline injector orifice size is a parameter that is controlled to extremely tight tolerances, pressure loss is a CNG, or other gaseous fuel, specific problem which must be considered in the overall design when using gaseous fuels in such injectors. Nevertheless, pressure loss is a natural phenomenon which occurs as fluid flows through any system. As the velocity of the fluid is increased and the fluid is forced through tortuous paths the losses can become quite substantial over the length of the path. These losses contribute directly to the loss of overall mass flow available from the injector. Without proper control of the high pressure loss areas in the injector, static flows would be nearly impossible to correlate.
The CNG injector generally has sonic flow exiting the injector. This occurs with CNG any time there is a 55% pressure differential across any given point in the system. While sonic choked flow is achieved, the downstream pressure is no longer included in the mass flow function. The only variables which contribute to the theoretical mass flow in a choked flow system are gas constants, upstream pressure, upstream temperature, and flow area. The gas constants for any given fuel passing through the injector from the fuel rail will be constant from injector to injector, and at present the area for the orifice is controlled very closely for gasoline applications. This leaves pressure and temperature as potential variables. The fuel temperature will not vary significantly from injector to injector due to the short time available for heat transfer. However, the pressure above the orifice is affected by all of the losses throughout the injector and may vary between injectors.
As the fuel flows from the fuel rail through the injector, each item comprising the flow path contributes to the total loss in pressure. Some of these losses are small and some are quite substantial. In the present CNG injector art, the main fuel path consists of the filter, upper inlet connector, adjusting tube, armature, valve body, lower guide, lower guide/seat masked area, needle/seat interface and lastly, the orifice.
The filter, upper inlet connector, adjusting tube, lower guide and valve body account for a very small portion of the overall pressure loss in the injector. The armature has a small intentional loss to allow for faster breakaway and dampening during opening impact of the valve needle. This leaves only the lower guide/seat interface and the needle/seat interface as the main controllable limiting factors for controlling pressure losses.
Theoretically, the needle/seat interface can be controlled through seat angle, spherical needle radius and lift. An increase in lift would reduce the magnetic force of the solenoid coil and lengthen the opening time and linearity of the injector. As the spherical radius of the needle increases, it thereby increases the exposed area for a given lift with the result that the net force of the gas pressure increases. This also lengthens the opening time of the injector. Presently such injectors utilize a needle/seat angle of approximately 90xc2x0. If the seat angle is increased from the present 90xc2x0 angle, the flow area exposed for a given lift also increases as long as the needle spherical radius is changed to accommodate the reduced sealing diameter. This concept, although appearing relatively simple, has several serious drawbacks.
When the seat angle is increased, two problems occur. The first problem is that the increased seat angle is more difficult to grind on existing seat grinding equipment. A good compromise between grinding capabilities and design can be reached to reduce the effect of this problem. The second problem is that the flow past the lower needle guide/seat interface becomes pinched and the flow loss from this interface becomes significant. The present invention provides significant flow control while avoiding the loss of fuel flow through a novel valve structure which incorporates a novel valve needle seat.
An electromagnetically operable fuel injector for a gaseous fuel injection system of an internal combustion engine is disclosed, the injector having a generally longitudinal axis, which comprises, a ferromagnetic core, a magnetic coil at least partially surrounding the ferromagnetic core, an armature magnetically coupled to the magnetic coil and being movably responsive to the magnetic coil, the armature actuating a valve closing element which interacts with a fixed valve seat of a fuel valve and being movable away from the fixed valve seat when the magnetic coil is excited. The fixed valve seat of the fuel valve defines a central fuel opening and a generally annular groove adjacent the central fuel opening, the armature having a generally elongated shape and a generally central opening for axial reception and passage of gaseous fuel from a fuel inlet connector positioned adjacent thereto. The fuel inlet connector and the armature are adapted to permit a first flow path of gaseous fuel between the armature and the magnetic coil as part of a path leading to the fuel valve.
In a preferred embodiment an electromagnetically operable fuel injector for a compressed natural gas fuel injection system of an internal combustion engine is disclosed, the injector having a generally longitudinal axis, which comprises, a ferromagnetic core, a magnetic coil at least partially surrounding the ferromagnetic core, an armature magnetically coupled to the magnetic coil and movably responsive to the magnetic coil, the armature having a first upper end face and a lower end portion. A valve closing element is connected to the lower end portion of the armature and is interactive with a fuel valve having a fixed valve seat to selectively permit fuel to pass through the valve seat as the valve closing element is moved to a valve open position by the armature. The fixed valve seat has a generally frusto-conically shaped portion surrounded by an adjacent circular shaped annular groove to reduce the pressure differential occurring across the valve closing element and the fixed valve seat upon closing the fuel valve. A fuel inlet connector extends in a generally longitudinal direction above the armature and defines a path for fuel to enter the inlet connector and to be directed toward the armature, the fuel inlet connector having a lowermost end portion having a lowermost surface spaced above the armature to define a working gap through which the armature is movable. The armature has a fuel reception portion for receiving fuel directed from the fuel inlet connector, and further defines a generally axial fuel passage.
A method of directing gaseous fuel through an electromagnetically operable fuel injector for a fuel system of an internal combustion engine is also disclosed, the injector having a generally longitudinal axis, and including a fuel inlet end portion and a fuel outlet end portion. A fuel inlet connector is positioned at the fuel inlet end portion and has a fuel inlet end portion and a fuel outlet end portion. An armature is positioned adjacent the fuel outlet end portion of the fuel inlet connector, the armature being spaced from the fuel inlet connector to define a working gap to permit movement of the armature toward and away from the fuel inlet connector to selectively open and close a fuel valve by providing upward and downward movement of a valve closing element to selectively permit gaseous fuel to pass therethrough to an air intake manifold. The method comprises directing the gaseous fuel to pass axially through the fuel inlet connector, directing the gaseous fuel to pass from the fuel inlet connector to the generally elongated central opening of the armature in an axial direction toward the fuel valve, and providing an annular groove adjacent the fixed valve seat for reception of fuel so as to reduce pressure losses across the fuel valve during closure thereof. In particular, the fuel is permitted to enter in volumetric space adjacent the fuel valve to reduce the pressure losses thereacross during closure of the fuel valve.